1. Field of the Invention
The invention relates to a method of fabricating a solid-state image sensor.
2. Description of the Related Art
Contaminants such as heavy metals may penetrate a wafer while a solid-state image sensor is being fabricated. In order to remove such contaminants out of a device active region, there is usually carried out gettering.
In one of gettering techniques, bulk-micro-defect (BMD) is formed in a wafer in an amount required for removing contaminants. That is, BMD is formed in heat treatment carried out in a process of fabricating a solid-state image sensor, and contaminants are gettered into the thus formed BMD. Accordingly, it is a key in gettering to form BMD in a wafer.
One of methods of forming BMD in a wafer includes the steps of slicing Cz (Czochralski-Zone)-Si ingot into wafers, polishing Si wafer at a surface thereof, carrying out first heat treatment at a temperature in the range of 1150 to 1200 degrees centigrade to thereby melt oxygen cores having been precipitated at a surface of a wafer, and externally diffuse oxygen from a surface of a wafer, and carrying out second heat treatment at a temperature in the range of 500 to 800 degrees centigrade to thereby form oxygen precipitation cores in a wafer.
In accordance with the above-mentioned method, oxygen precipitation cores are not formed at a surface of a wafer in the second heat treatment, since oxygen cores having been precipitated at a surface of a wafer is molten, and oxygen is externally diffused from a surface of a wafer in the first heat treatment. As a result, a wafer can have a non-defective layer at a surface thereof, and can contain oxygen precipitation cores therein.
After oxygen precipitation cores have been formed in a wafer in the above-mentioned way, the wafer is subject to heat treatment at a temperature in the range of 1000 to 1200 degrees centigrade to thereby cause oxygen to be precipitated out of oxygen precipitation cores. Thus, there is formed BMD.
However, the above-mentioned method of forming BMD is accompanied with a problem that a solid-state image sensor including a wafer fabricated in accordance with the above-mentioned method would have illuminated or white defects at a high level. This is considered because contaminants such as heavy metals which penetrate a wafer while a solid-state image sensor is being fabricated are insufficiently gettered into BMD in a wafer, and in particular, because BMD is insufficiently grown in a wafer.
Apart from the above-mentioned method, various gettering techniques have been suggested as follows in order to remove contaminants such as heavy metals out of a device active region.
For instance, Japanese Unexamined Patent Publication No. 57-35329 has suggested a method including the steps of precipitating oxygen atoms in a bulk at a temperature in the range of 1150 to 1200 degrees centigrade to thereby remove defects having been formed at a surface of a wafer, and growing oxygen precipitation at a temperature in the range of 1000 to 1100 degrees centigrade to thereby enhance gettering effect.
Japanese Unexamined Patent Publication No. 1-242500 has suggested a method including the steps of externally diffusing oxygen from a surface of a wafer at 1200 degrees centigrade to thereby diffuse silicon existing at a surface of a wafer, into a wafer, and cooling the wafer down to a temperature in the range of 500 to 800 degrees centigrade at a cooling rate of 1 to 10 degrees per minute, to thereby form oxygen cores in a bulk.
Japanese Unexamined Patent Publication No. 4-43646 has suggested a method of growing an epitaxial layer on a silicon substrate fabricated in accordance with Cz process, and carrying out both heat treatment at a relatively low temperature for a shorter period of time and heat treatment at a relatively high temperature for a longer period of time to thereby facilitate precipitation of oxygen in the Cz substrate.
Japanese Unexamined Patent Publication No. 5-74782 has suggested a method of carrying out intrinsic gettering by applying heat treatment at three stages to a silicon substrate sliced out of silicon monocrystal having been formed in accordance with Cz process. The first heat treatment is carried out at a temperature in the range of 1100 to 1150 degrees centigrade, the second heat treatment is carried out at a temperature equal to or higher than 1200 degrees centigrade, and the third treatment is carried out at a temperature lower than a temperature at which the first heat treatment was carried out.
Japanese Unexamined Patent Publication No. 9-199379 has taught that heat treatment to be carried out at a temperature in the range of 1000 to 1200 degrees centigrade is optimal for obtaining a high grade epitaxial wafer.
However, the inventor has analyzed the methods suggested in the above-mentioned Publications to thereby find out that it would be quite difficult or almost impossible to sufficiently grow BMD in a wafer even in accordance with the above-mentioned methods, and that it is not always possible to obtain desired gettering effect.